In recent years, research on the Wavelength Division Multiplexing (WDM) transmission is very active and a lot of investigations are also performed to obtain an optical fiber suitable for this WDM transmission. The optical fiber used for the WDM transmission is required to have a low loss in the whole wavelength range to be used. Though, up to now, the WDM transmission has been examined around 1.55 μm band, wherein the transmission loss of the silica glass optical fiber is the lowest, the further expansion of the transmission capacity is demanded. Therefore, attempts are made to expand the used wavelength range to 1310 nm-1625 nm.
On the other hand, the OH group, which causes the absorption peak around 1385 nm, was often included in a conventional optical fiber as impurities. The optical fiber including these impurities shows a large transmission loss around 1385 nm.
In addition, when hydrogen diffuses in the optical fiber, it is known that the transmission loss around the wavelengths of 1240 nm, 1385 nm, 1530 nm or the wavelength range longer than 1530 nm etc. are increasing. These increases of the transmission loss are based on an increase of the absorption peak owing to the OH group.
It is necessary to reduce these absorption peaks as much as possible in order to expand the wavelength range of the WDM transmission to 1310 nm-1625 nm.
The phenomenon of the increase of transmission loss by the hydrogen diffusion to the optical fiber is generated by the following mechanism.
In general, the structural defect, which is called an oxygen-excessive or oxygen-deficient paramagnetic defect, exists in the optical fiber. It is known that, among the paramagnetic defects, the Non-Bridging Oxygen Hole Center (NBOHC) has an especially big influence on the increase of the transmission loss of the optical fiber.
When hydrogen diffuses in the optical fiber, a diffused hydrogen molecule reacts with a paramagnetic defect and creates an atomic bond having absorption in the transmission range of the optical fiber, and an increase of the transmission loss occurs.
Concretely, when a paramagnetic defect is the NBOHC, the OH group is generated by the reaction shown by the following expression, and the OH absorption loss is increased.2≡Si—O.+H2→2≡Si—OH . . .   (1)
Here, Si—O. is the NBOHC. Moreover “.” shows unpaired electron and “≡” shows the covalent bond with three oxygen atoms.
Especially, when many NBOHCs exist in a Mode Field Diameter (MFD) of the optical fiber, an increase of the OH absorption loss is remarkable, and it increases with time as hydrogen diffuses.
Therefore, it is necessary to decrease the NBOHC, especially the NBOHC in the MFD, to improve the hydrogen-proof characteristic of the optical fiber.
The optical fiber, in which the OH absorption loss is controlled and which has a refractive index profile similar to the Single Mode Fiber (SMF), is proposed in the U.S. PTO. No.6,131,415.
In the optical fiber proposed in this patent, it is supposed that optical transmission throughout the wavelength range of 1200-1600 nm becomes possible.
Moreover, it is shown that the NBOHC causes absorption around the wavelength of 630 nm in (Y.Hibino and H.Honafusa,J.Appl. Phys.,60,1797.(1986), etc).
On the other hand, the method of exposing the optical fiber to hydrogen or deuterium during the drawing process or after the drawing is proposed as the method to decrease the NBOHC density in the optical fiber (Refer to Japanese Laid-Open Patent Application (JP-A;KOKAI) No.Hei7-277770).
However, when the hydrogen ageing is carried out, above-mentioned reaction of the expression (1) occurs. And, though the NBOHC disappears, on the other hand, there is a problem that the OH absorption loss increases.
Therefore, in order to decrease the NBOHC density, the deuterium ageing is suitable because an increase of the OH absorption loss doesn't occur accompanying to the disappearance of the NBOHC.
The reaction shown by the following expression takes place in the optical fiber when the deuterium ageing is carried out.2≡Si—O.+D2→2≡Si—OD . . .   (2)
That is, the OD group is generated, and the OH absorption doesn't occur. Since the OD group doesn't have any large absorption peak in 1310 nm-1625 nm, there is little influence on the transmission loss in this wavelength range.
Moreover, so far it was the only method of evaluating the degree of a loss increase when the optical fiber was exposed to hydrogen (hereafter it is called the hydrogen-proof characteristic) wherein a part of the product was extracted as a sample to which the hydrogen ageing was carried out to confirm the presence of the problem.
Especially, the hydrogen ageing condition as shown below is provided, for instance, by IEC60793-2 Amendment 1,2001-8 Annex C recently.
The optical fiber is exposed to the atmosphere containing hydrogen of 1 volume %, i.e., 0.01 atmospheres at room temperature until the transmission loss increases 0.03 dB/km or more at 1240 nm compared with the loss of exposing before (the initial loss), and the transmission loss is measured after leaving it in open atmosphere for more than 14 days.